The present invention relates to fuel cells that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to a fuel cell power plant that utilizes fuel processing components to produce a hydrogen enriched reformate fuel from a hydrocarbon fuel.
Fuel cell power plants are well-known and are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus such as apparatus on-board space vehicles. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids. Each individual cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reactant or reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane (xe2x80x9cPEMxe2x80x9d) as the electrolyte, the hydrogen electrochemically reacts at a surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is the aforesaid proton exchange membrane (xe2x80x9cPEMxe2x80x9d) electrolyte, which consists of a solid polymer well-known in the art. Other common electrolytes used in fuel cells include phosphoric acid or potassium hydroxide held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials than a liquid electrolyte held by capillary forces within a porous matrix. Additionally, the PEM electrolyte is fixed, and cannot be leached from the cell, and the membrane has a relatively stable capacity for water retention.
Manufacture of fuel cells utilizing PEM electrolytes typically involves securing an appropriate first catalyst layer, such as a platinum alloy, between a first surface of the PEM and a first or anode porous substrate or support layer to form an anode electrode adjacent the first surface of the PEM, and securing a second catalyst layer between a second surface of the PEM opposed to the first surface and a second or cathode porous substrate or support layer to form a cathode electrode on the opposed second surface of the PEM. The anode catalyst, PEM, and cathode catalyst secured in such a manner are well-known in the art, and are frequently referred to as a xe2x80x9cmembrane electrode assemblyxe2x80x9d, or xe2x80x9cM.E.A.xe2x80x9d, and will be referred to herein as a membrane electrode assembly. In operation of PEM fuel cells, the membrane is saturated with water, and the anode electrode adjacent the membrane must remain wet. As hydrogen ions produced at the anode electrode transfer through the electrolyte, they drag water molecules in the form of hydronium ions with them from the anode to the cathode electrode or catalyst. Water also transfers back to the anode from the cathode by osmosis. Product water formed at the cathode electrode is removed from the cell by evaporation or entrainment into a gaseous stream of either the process oxidant or reducing fluid.
A fuel cell power plant includes a fuel cell or fuel cell stack to generate electricity and a variety of systems to support the fuel cell stack. For example, if the plant is to be utilized to power a transportation vehicle, it is necessary that the power plant be self-sufficient in water to be viable. Self-sufficiency in water means that enough water must be retained within the plant to offset losses from reactant fluids exiting the plant in order to efficiently operate the plant. Any water exiting the plant through a plant process exhaust stream consisting of a cathode exhaust stream of gaseous oxidant and/or an anode exhaust stream of fluid exiting the anode side of the fuel cell must be balanced by water produced electrochemically at the cathode electrode and water retained within the plant. To maintain water self-sufficiency, it is common that the plant include a water recovery device, controls, and piping to recover and direct water into the fuel cell stack to maintain proper wetting of the PEM electrolytes, and humidity of the reactant streams, etc. An additional known component that assists in maintaining water balance is a water transport cooler plate secured in fluid communication with the cathode electrode or catalyst so that product water generated electrochemically at the cathode catalyst may move into the cooler plate to mix with a cooling fluid passing through the plate and then be directed to other plant systems.
Additionally, it is known that some fuel cell power plants operate on pure hydrogen gas, while others utilize a reformate fuel wherein a hydrogen enriched reducing fluid is formed from any of a variety of hydrocarbon fuels by fuel processing components including for example use of known autothermal, steam or partial oxidation reformers. Unfortunately, such reformation of hydrocarbon fuels generates ammonia that moves with the reformate fuel gas reactant stream into the fuel cell where the ammonia dissolves in the water in the electrolyte to become ammonium ions. The ammonia is formed in the reformer by a reaction between hydrogen and nitrogen present in the air that is used in the reforming process or nitrogen added to a peak shaved natural gas. The ammonium ions are then adsorbed by the PEM electrolyte to displace protons within the PEM, thereby decreasing conductivity of the PEM, and hence having a significant negative effect on performance of the fuel cell. Depending upon the temperature of the reformer, composition of any catalyst in the reformer, and nitrogen concentration within the reformer, ammonia formed in the reforming process may range from 1-100 parts per million (xe2x80x9cppmxe2x80x9d). To efficiently operate a fuel cell power plant on such reformate fuel, the ammonia must be effectively removed from the fuel prior to entry of the fuel into the fuel cells of the plant.
Accordingly, there is a need to develop a fuel cell power plant that includes a reformate fuel treatment system for producing a reformate fuel with ammonia contamination less than 1.0 ppm.
The invention is a reformate fuel treatment system for a fuel cell power plant that includes at least one fuel cell for generating electricity from process oxidant and reducing fluid reactant streams; fuel processing components including a steam supply, a reformer and a water shift reactor of converter for producing a hydrogen enriched reformate fuel for the fuel cell from a hydrocarbon fuel; and, an ammonia removal apparatus that treats the reformate fuel to make it appropriate for supplying hydrogen to an anode electrode of the fuel cell. I n one embodiment of the reformate fuel treatment system, the ammonia removal apparatus is a disposable ammonia scrubber including a bed of carbon pellets saturated with phosphoric acid, a molecular sieve such as alumina or zeolites, or a cation exchange resin. Additionally, the reformer that directs the reformate fuel to the disposable ammonia removal scrubber may receive steam from a burner and steam generator in fluid communication with the fuel cell, wherein the burner receives and combusts an anode exhaust stream exiting the fuel cell, and the steam generator receives water from a water transport cooler plate within the fuel cell in fluid communication with a cathode catalyst of the fuel cell.
In an alternative embodiment of the reformate fuel treatment system, the ammonia removal apparatus includes an ammonia scrubbing cool water bed and an ammonia stripping warm water bed, wherein the reformate fuel passes through the cool water bed to have any ammonia removed from the reformate fuel into the water prior to the fuel passing into the fuel cell. The water then cycles from the cool water bed through a heat exchanger in heat exchange relationship with a coolant fluid exiting the fuel cell, and then enters the warm water bed where an oxidant stream passes through the bed to strip the ammonia from the water by oxidizing the ammonia to nitric oxides, or by simply stripping it, before the water is cooled and returned to the warm water bed. Alternatively, temperatures of the cool water bed and warm water bed may be controlled using coolant fluids from other components of the fuel cell power plant.
In an additional embodiment of the reformate fuel treatment system, the ammonia removal apparatus includes a pair of first and second regenerable scrubbers, wherein the reformate fuel passes through a first regenerable scrubber and any ammonia in the fuel is removed and the reformate fuel is then directed to the fuel cell. When the first regenerable scrubber is no longer able to remove adequate amounts of ammonia from the reformate fuel, the fuel is directed to pass through a second regenerable scrubber prior to passing into the fuel cell, and simultaneously an oxidant stream is directed to pass through the first regenerable scrubber to regenerate the scrubber by oxidizing the ammonia to nitrogen or nitric oxides. The oxidant stream may be a cathode exhaust stream passing out of the fuel cell. Whenever the second regenerable scrubber is unable to remove adequate amounts of ammonia from the reformate fuel, the fuel is directed to pass through the regenerated first regenerable scrubber, while the second regenerable scrubber is regenerated by the oxidant stream passing through the second regenerable scrubber. As with the disposable scrubber embodiment, the reformer that directs the reformate fuel to the regenerable scrubbers may receive steam from the burner and steam generator.
In use of the reformate fuel treatment system for a fuel cell power plant, if the fuel cell power plant is utilized to power a transportation vehicle, such as an automobile having a limited annual operating cycle of approximately 500 hours, the disposable ammonia scrubber may be adequate to treat the reformate fuel to acceptable minimum levels of less than 1.0 parts per million (xe2x80x9cppmxe2x80x9d). Use of the ammonia scrubbing cool water bed and ammonia stripping warm water bed embodiment, or of the pair of regenerable scrubbers embodiment of the reformate fuel treatment system provides for ongoing, uninterrupted operation of the power plant. Where the fuel cell power plant is powering a transportation vehicle and steam for the reformer is supplied through the steam generator from water generated by the fuel cell, the reformate fuel treatment system also supports operation of the plant in water balance wherein water generated electrochemically by the fuel cell is retained and efficiently utilized within the power plant.
Accordingly, it is a general object of the present invention to provide a reformate fuel treatment system for a fuel cell power plant that overcome deficiencies of the prior art.
It is a more specific object to provide a reformate fuel treatment system for a fuel cell power plant that reduces ammonia contamination of the reformate fuel to an acceptable level.
It is yet another object to provide a reformate fuel treatment system for a fuel cell power plant that reduces ammonia contamination of the reformate fuel to an acceptable level without interruption of operation of the power plant.
It is another object to provide a reformate fuel treatment system for a fuel cell power plant that reduces ammonia contamination of the reformate fuel to an acceptable level and that assists operation of the power plant in water self-sufficiency.
These and other objects and advantages of the reformate fuel treatment system for a fuel cell power plant will become more readily apparent when the following description is read in conjunction with the accompanying drawings.